The ability to create, support, propagate and detect single walled magnetic domains in particular types of material has been applied, in the prior art, to perform memory and logic functions, useful, for example, in digital computer type applications. The single walled magnetic domains, or more popularly "bubbles" (as they will be referred to herein) are actually regions of magnetization, in a particular type of material which is directly opposite to the magnetization direction of a major portion of the material. Regardless of the function to be performed, it is well known that such bubble devices include at least the following, a sheet of material which is capable of supporting bubbles under the proper conditions, adjacent such a sheet of material are provided some means for, at least, propagating the bubbles. The most popular arrangement of apparatus for propagating the bubbles is termed "field access" and includes properly shaped elements of a magnetic material which, when subjected to a reorienting drive magnetic field parallel to the plane of the sheet of material supporting the bubbles, produce a propagating series of potential wells which cause the bubble or bubbles in the sheet of material supporting the bubbles to propagate synchronously with the potential wells. A further bias field is also provided normal to the magnetic sheet of material and the interaction of the bias and drive fields provide for stable propagating bubbles along the paths defined by the magnetic elements. the reorienting magnetic field usually comprises a magnetic field rotating about an axis parallel to the bias field.
The prior art has disclosed a number of devices for the selective switching of the propagation path followed by a stream of bubbles. One class of such devices includes a conductor arranged to cross a bubble propagation path such that the presence or absence of current in the conductor determines which of the possible propagation paths a bubble will follow.
The fabrication of bubble devices has employed some of the same fabrication techniques employed for integrated circuits, namely, the selective deposition and/or etching of the material through masks of desired patterns. One particularly advantageous procedure is termed "single level masking" (hereinafter SLM) in which a single masking step defines at least the critical patterns of the device. Accordingly, the same critical patterns are composed of the same material. An example of a bubble transfer switch which can be fabricated employing SLM techniques is disclosed in the co-pending application of George S. Almasi, W. E. Bogholtz and G. E. Keefe, filed July 28, 1976, Ser. No. 709,358.
Of particular importance in the fabrication and operation of bubble devices is the tolerance those devices exhibit to changes in bias and drive field, and one figure of merit for these devices is the range of field amplitudes over which they will operate, sometimes called margins. It has been found that the margins of SLM bubble devices in which a conductor crosses a field access bubble propagation path is more limited than the operating margins for the field access bubble propagation path itself. More particularly, bubble collapse has been noted in the region of the conductor. It has been found particularly difficult to arrange bubble transfer devices which include a conductor crossing a field access propagation path fabricated using SLM technology to reliably operate with desirably small bubbles, although the reasons for the difficulty are not clearly understood.
This particular difficulty becomes pronounced when one desires to work with bubbles whose diameter is on the order of one micron or less.
In addition, bubble devices which are not necessarily fabricated with SLM techniques, are subject to bubble stripout or collapse. Bubble stripout can occur in a Y-bar bubble switch, for instance, between an element of the switch and either another element of the switch or an adjacent element. Bubble collapse is noted during bubble travel through the switch. Both problems are related to limited operating margins, the former relates to limited lower field margins and the latter to limited upper field margins.
It is therefore one object of the invention to provide a method and apparatus for increasing operating margins in field access bubble devices. It is another object of the invention to provide method and apparatus for increasing operating margins in bubble switches in which bubble travel traces out a Y. It is another object of the invention to provide in SLM field access bubble devices a method and apparatus to overcome the heretofore encountered difficulties in the vicinity of a conductor. It is another object of the invention to provide a method of operating such a device which allows increased operating margins especially in the region of the crossing of the conductor and the field access propagation path. It is still another object of the invention to provide an improved bubble transfer switch with increased operating margins at least in the region wherein a conductor crosses a field access bubble propagation path.